#include<bits/stdc++.h>
using namespace std;
const int maxm = 110;
int n, m, lb;
int k[maxm], a[maxm], b[maxm];
int main() {
  cin>>n>>m;
  cin.get(); // dummy
  lb = m-n;
  for(int i=lb; i<m; ++i) {
    a[i] = cin.get()-'a';
  }
  cin.get();
  for(int i=0; i<m; ++i) {
    b[i] = cin.get()-'a';
  }
  while(lb>n) {
    for(int i=lb; i<lb+n; ++i) {
      k[i] = (b[i]-a[i]+26)%26; 
    }
    memcpy(a+lb-n, k+lb, n*sizeof(a[0]));
    lb-=n;
  }
  lb--;
  while(lb>=0) {
    k[lb+n] = (b[lb+n]-a[lb+n]+26)%26;
    a[lb] = k[lb+n];
    lb--;
  }
  for(int i=0; i<m; ++i) {
    cout<<char(a[i]+'a');
  }
  cout<<'\n';
}